The process of manufacturing three-piece metal cans typically involves forming a cylindrical can body from a sheet of precoated metal and then attaching two precoated lids to the opposite ends of the can body. In constructing a cylindrical can body, a sheet or blank of metal is formed around a mandrel or stubhorn. The edges of the sheet are either butted or overlapped and then secured together by welding. The welded seam is covered with a protective coating to protect the contents of the can, e.g. food, against metal contamination. The welded seams are coated with either a liquid or powder coating. If a powder coating is used, oversprayed powder is typically recovered from within the container bodies by a vacuum device.
After covering the seam with the protective coating, the can is subjected to heat for a preset period of time to cure the coating material. Where powder coating is used, it is important that a uniform layer of the powder coating, typically a dry resin powder, is deposited on the seam so that the powder melts and fuses to form a smooth adherent coating within the preset period of curing time. If the coating is too thick in some area, it might not completely fuse within the curing time and can later contaminate the contents of the can.
According to the prior art, a coating of liquid material is typically applied to the interior welded seam of a can body as disclosed, for example, in U.S. Pat. Nos. 3,526,027, 4,215,648, 4,259,923 and 4,346,667, with a coating gun attached directly to the end of the welding arm or stubhorn. Other patents which teach the application of coating powder onto a welded seam of a can body with a powder applicator that is secured to the end of the welding arm include U.S. Pat. Nos. 4,215,648, 4,259,923 and Re 33,394.
It is desirable to coat the entire interior of the can with powder to avoid solvent emissions given off from can blanks which are precoated. In addition, liquid coatings typically must be applied in thin layers requiring the application of two coats with a curing step after each coat. This process requires a high amount of energy utilization since two curing ovens, or two passes through a single curing oven, are required. Powder coatings, on the other hand, have no solvent emissions and can be uniformly applied and cured in one step as a thicker coating. Other advantages can also be obtained by coating the interior surface of cans on the welding arm of a can forming machine powder. Up to the present time, however, the industry has not satisfactorily developed commercially suitable equipment to powder coat cans at the end of a welding arm.
U.S. Pat. No. 4,343,436, describes applying a coating of powder to the interior of a hollow can with a spray gun mounted on the end of a welding arm so that a thicker coating is applied to the welded seam as compared to the remainder of the interior surface of the can. This concept, as disclosed in the U.S. Pat. No. 4,343,436 , is deficient, for example, because it does not describe how the amount of air-entrained powder needed to coat the entire interior surface of the can be transferred through the narrow passage of the welding arm at satisfactory flow rates without excessive transport air which then has to be removed from the interior of the can.